LibJS: Resolve a circular include problem between HeapBlock and Cell

Cell::heap() and Cell::vm() needed to access member functions from
HeapBlock, and wanted to be inline, so they were moved to VM.h.
That approach will no longer work with VM.h not being included in every
file (starting from the next commit), so this commit fixes that circular
import issue by introducing secondary base classes to host the
references to Heap and VM, respectively.

Userland/Libraries/LibJS/Heap/Internals.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2020-2023, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+#include <LibJS/Forward.h>
+
+namespace JS {
+
+class HeapBase {
+    AK_MAKE_NONCOPYABLE(HeapBase);
+    AK_MAKE_NONMOVABLE(HeapBase);
+
+public:
+    VM& vm() { return m_vm; }
+
+protected:
+    HeapBase(VM& vm)
+        : m_vm(vm)
+    {
+    }
+
+    VM& m_vm;
+};
+
+class HeapBlockBase {
+    AK_MAKE_NONMOVABLE(HeapBlockBase);
+    AK_MAKE_NONCOPYABLE(HeapBlockBase);
+
+public:
+    static constexpr auto block_size = 16 * KiB;
+    static HeapBlockBase* from_cell(Cell const* cell)
+    {
+        return reinterpret_cast<HeapBlockBase*>(bit_cast<FlatPtr>(cell) & ~(HeapBlockBase::block_size - 1));
+    }
+
+    Heap& heap() { return m_heap; }
+
+protected:
+    HeapBlockBase(Heap& heap)
+        : m_heap(heap)
+    {
+    }
+
+    Heap& m_heap;
+};
+
+}